1. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to a single polarization and polarization maintaining optical fibers.
2. Technical Background
Single polarization optical fibers and polarization maintaining fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with and connection to optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). Single polarization fiber propagates one, and only one, of two orthogonally polarized polarizations within the single polarization band while suppressing the other polarization by increasing its transmission loss at specific wavelength range. Such single polarization fibers generally have an azimuthal asymmetry of the refractive index profile. The polarization maintaining fiber maintains two orthogonal polarization modes in an operating wavelength range.
One type of prior polarization retaining fiber includes, as shown in FIG. 1, a central core 10 surrounded by an inner cladding region 11. Core 10 and cladding region 11 are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material must be greater than that of the cladding material and that both materials must exhibit low losses at the wavelength at which the waveguide is intended to be operated. By way of example only, core 10 may include silica containing one or more dopants which increase the refractive index thereof, such as germania. Region 11 may comprise pure silica, silica containing a lesser amount of dopant than core 10, or silica containing one or more down dopants, at least one of which is an oxide of an element such as boron or fluorine which lowers the refractive index of silica. Polarization maintaining fiber has high birefringence (Δn), which is defined by the difference of effective refractive index between the two polarizations of the fundamental mode. The light belonging to each state of polarization travels at a slightly difference velocity in the fiber at a given wavelength λ. The birefringence determines the distance required to accumulate one wavelength of phase delay between the two polarization modes, which is referred to as beatlength LB. The fiber beatlength LB is related to the fiber birefringence (Δn) in a simple equation,LBλ/Δn.The strength of the polarization maintaining capability of the fiber is related to the fiber birefringence or beatlength. The shorter the beatlength, the higher the birefringence and therefore the better the state of polarization is maintained. It can be found from the above equation that the beatlength is linearly proportional to the wavelength, which means that for a fixed birefringence Δn, the beatlength increases proportionally with the wavelength λ.
In FIG. 1, diametrically opposed relative to core 10, are two regions 12 formed of a glass material having a Coefficient of Thermal Expansion (TCE) different from that of cladding material 11. When such a fiber is drawn, the longitudinally-extending regions 12 and the cladding regions disposed orthogonally thereto will shrink different amounts whereby regions 12 will be put into a state of tension or compression depending upon the TCE thereof relative to that of the cladding 11. A strain induced birefringence, which is thus induced in the fiber, reduces coupling between the two orthogonally polarized fundamental modes. Surrounding regions 12 is an outer cladding region 13, the refractive index of which is preferably equal to or less than that of inner cladding region 11. Region 13 may consist, for example, of any of the materials specified above for use as cladding region 11.
Slight improvement in the polarization performance of single mode optical waveguides has been achieved by elongating or distorting the fiber core symmetry as a means of decoupling the differently polarized waves. Examples of such optical fiber waveguides with elongated cores are disclosed in U.S. Pat. Nos. 4,184,859, 4,274,854, and 4,307,938. FIG. 2 herein illustrates a waveguide 1 having a core 4 having refractive index n1, a cladding 5 having a refractive index n2, wherein the elongated core 4 has a major axis a and a minor axis b. However, the noncircular geometry and the associated stress-induced birefringence alone are, generally, not sufficient to maintain the desired single polarization.
It has, therefore, been an area of ongoing development to obtain a fiber providing single polarization and polarization maintaining characteristics.